fix(isFloat): reject sign-only + decimal-separator combinations

[JSap0914]

Problem

isFloat('+.') and isFloat('-.') return true but should return false. The same false positive appears with comma-decimal locales (+,/-, for de-DE) and with the Arabic decimal separator (+٫/-٫ for ar-JO).

Closes #2090

Root cause

The early-return blacklist in src/lib/isFloat.js rejects each problematic token individually:

if (str === '' || str === '.' || str === ',' || str === '-' || str === '+') {
  return false;
}

But the combined form — a sign immediately followed by the decimal separator and nothing else — slips through because it is not in the list. The float regex then matches it (the digit groups are all optional) and returns true, while parseFloat('+.') returns NaN, so the result is semantically wrong.

Fix

Extend the early-return to also reject the two combined forms (+<sep> and -<sep>) for the dot and comma separators, and derive the locale's actual separator (e.g. ٫ for Arabic locales) to cover non-ASCII decimal characters too:

+ const decimalSep = options.locale ? decimal[options.locale] : '.';
- if (str === '' || str === '.' || str === ',' || str === '-' || str === '+') {
+ if (str === '' || str === '.' || str === ',' || str === '-' || str === '+'
+   || str === '+.' || str === '-.' || str === '+,' || str === '-,'
+   || str === `+${decimalSep}` || str === `-${decimalSep}`) {

Verification

$ npx mocha --reporter dot --require @babel/register --recursive test
  318 passing (596ms)

Six new invalid fixtures were added (two per locale: default ., de-DE comma, ar-JO Arabic separator). All previously-valid inputs (+.123, -.5, +,123 for de-DE, etc.) remain valid. No existing tests were modified.

This fix was developed with AI assistance.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (3d2f4b3) to head (d646e37).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2786   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          114       114           
  Lines         2587      2588    +1     
  Branches       656       657    +1     
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+ Hits          2587      2588    +1     

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[WikiRik]

Why not a regular expression as suggested in the issue?